Improved modeling of IEEE 802.11a PHY through fine-grained measurements

In wireless networks, modeling of the physical layer behavior is an important yet difficult task. Modeling and estimating wireless interference is receiving great attention, and is crucial in a wireless network performance study. The physical layer capture, preamble detection, and carrier sense threshold are three key components that play important roles in successful frame reception in the presence of interference. Using our IEEE 802.11a wireless network testbed, we carry out a measurement study that reveals the detailed operation of each component and in particular we show the terms and conditions (interference timing, signal power difference, bitrate) under which a frame survives interference according to the preamble detection and capture logic. Based on the measurement study, we show that the operations of the three components in real IEEE 802.11a systems differ from those of popular simulators and present our modifications of the IEEE 802.11a PHY models to the NS-2 and QualNet network simulators. The modifications can be summarized as follows. (i) The current simulators’ frame reception is based only on the received signal strength. However, real 802.11 systems can start frame reception only when the Signal-to-Interference Ratio (SIR) is high enough to detect the preamble. (ii) Different chipset vendors implement the frame reception and capture algorithms differently, resulting in different operations for the same event. We provide different simulation models for several popular chipset vendors and show the performance differences between the models. (iii) Based on the 802.11a standard setting and our testbed observation, we revise the simulator to set the carrier sense threshold higher than the receiver sensitivity rather than equal to the receiver sensitivity. We implement our modifications to the QualNet simulator and evaluate the impact of PHY model implementations on the wireless network performance; these result in an up to six times increase of net throughput.     

Automated wireless video surveillance: an evaluation framework

In the past years, surveillance systems have attracted both industries and researchers due to its importance for security. Automated Video Surveillance (AVS) systems are established to automatically monitor objects in real-time. Employing wireless communication in an AVS system is an attractive solution due to its convenient installation and configuration. Unfortunately, wireless communication, in general, has limited bandwidth, not to mention the intrinsic dynamic conditions of the network (e.g., collision and congestion). Many solutions have been proposed in the literature to solve the bandwidth allocation problem in wireless networks, but much less work is done to design evaluation frameworks for such solutions. This paper targets the demand for a realistic wireless AVS system simulation framework that models and simulates most of the details in a typical wireless AVS framework. The proposed simulation framework is built over the well-known NS-3 network simulator. This framework also supports the testing and the evaluation of cross-layer solutions that manages many factors over different layers of AVS systems in the wireless 802.11 infrastructure network. Moreover, the simulation framework supports the collection of many used performance metrics that are usually used in AVS system performance evaluation.     

Information seeking and model predictive control of a cooperative multi-robot system

In this paper, we propose a cooperative multi-robot control system, operating in an unfamiliar or unstructured environment. We focus on a robust model predictive control (robust-MPC) framework that enables robotic agents to operate in uncertain environments, and study the effect of observation uncertainties that arise from sensor noise on cooperative control performance. The proposed system relies on cooperative observation based on an information-seeking theory, in which the system not only can compensate uncertainty, but also takes actions to mitigate it. We carry out a case study that demonstrates a multi-robot collision avoidance scenario in an unknown environment. Simulation results show that the combination of robust-MPC methods and cooperative observation enables the cooperative multi-robot system to move efficiently and reach the goal faster than an uncooperative scenario.     

基于J-Sim的网络仿真分析与设计

网络仿真技术成为目前研究网络算法﹑网络协议的一种主要手段.近年来随着网络规模的增大和复杂性的增加对网络仿真模拟器也提出了更高的要求.现有文献提出了很多方法来改进网络仿真平台以适应网络技术的发展.文中主要对一种新的网络仿真模拟器J-Sim的网络仿真原理进行了概述,并从不同的角度对NS-2和J-Sim网络仿真平台进行了对比,最后实例说明了J-Sim仿真的步骤并显示仿真结果.通过对比分析证明J-Sim是一种效率高,易于扩展的网络仿真模拟器,并通过一个实例验证.     

基于NS-3的MQTT协议仿真研究

如今,制造业数字化转型中采用消息队列遥测传输（MQTT）协议已成为趋势,而工业场景中往往存在大量的传感器等设备。考虑到实际部署的复杂性,在工业场景中实际部署MQTT前,进行必要的模拟以获取网络性能等数据是不可或缺的,这将更有利于工业场景中的链路设计和网络规划。而NS-3作为当前最流行的网络仿真器之一,提供了丰富的网络模型,非常适合像工业场景这种大型复杂网络的仿真,但目前利用其模拟部署MQTT仍是一项困难的工作。针对该问题,提出了一个适用于NS-3的扩展仿真框架ns3-mqtt,该框架由多个MQTT组件构成,能够以软件包的形式集成到NS-3中,并且提供简洁易用的仿真接口,使得扩展后的NS-3可较容易地对MQTT进行模拟部署并获取相关数据,以此研究不同工业场景下MQTT协议的性能,指导MQTT的实际部署。仿真测试结果证明了提出的ns3-mqtt框架的正确性和有效性。     

FluNet: A hybrid internet simulator for fast queue regimes

Motivated by the scale and complexity of simulating large-scale networks, recent research has focused on hybrid fluid/packet simulators, where fluid models are combined with packet models in order to reduce simulation complexity as well as to track dynamics of end-sources accurately. However, these simulators still need to track the queuing dynamics of network routers, leading to considerable simulation complexity in a large-scale network model. In this paper, we propose a new hybrid simulator – FluNet – where queueing dynamics are not tracked, but instead, an equivalent rate-based model is used. The FluNet simulator is predicated on a fast-queueing regime at bottleneck routers, where the queue length fluctuates on a faster time-scale than end systems. This allows us to simulate large-scale systems, where the simulation “time step-size” is governed only by the time-scale of the end-systems, and not by that of the intermediate routers; whereas a queue-tracking based fluid simulator would require decreasingly smaller step-sizes as the system scale size increases. We validate our model using a ns-2 based implementation. Our results indicate a good match between packet systems and the associated FluNet system.     

Declarative secure distributed information systems

We present a unified declarative platform for specifying, implementing, and analyzing secure networked information systems. Our work builds upon techniques from logic-based trust management systems and declarative networking. We make the following contributions. First, we propose the Secure Network Datalog (SeNDlog) language that unifies Binder, a logic-based language for access control in distributed systems, and Network Datalog, a distributed recursive query language for declarative networks. SeNDlog enables network routing, information systems, and their security policies to be specified and implemented within a common declarative framework. Second, we extend existing distributed recursive query processing techniques to execute SeNDlogprograms that incorporate secure communication via authentication and encryption among untrusted nodes. Third, we demonstrate the use of user-defined cryptographic functions for customizing the authentication and encryption mechanisms used for securing protocols. Finally, using a local cluster and the PlanetLab testbed, we perform a detailed performance study of a variety of secure networked systems implemented using our platform.     

Toward scalable cloud data center simulation using high-level architecture

Existing simulators are designed to simulate a few thousand nodes due to the tight integration of modules. Thus, with limited simulator scalability, researchers/developers are unable to simulate protocols and algorithms in detail, although cloud simulators provide geographically distributed data centers environment but lack the support for execution on distributed systems. In this paper, we propose a distributed simulation framework referred to as CloudSimScale. The framework is designed on top of highly adapted CloudSim with communication among different modules managed using IEEE Std 1516 (high-level architecture). The underlying modules can now run on the same or different physical systems and still manage to discover and communicate with one another. Thus, the proposed framework provides scalability across distributed systems and interoperability across modules and simulators.     

Modeling discrete event scalable network systems

Scalability in simulation tools is one of the most important traits to measure performance of software. The reason is that today’s Internet is the main instance of a large-scale and highly complex system. Simulation of Internet-scale network systems has to be supported by any simulation tool. Despite this fact, many network simulators lacks support for building large models. In this work, in order to propose a new approach for scalability issue in network simulation tools, a network simulator is developed based on behavior of honeybees and high performance DEVS, modular and hierarchical system theoretic approach. A biologically-inspired discrete event modeling approach is described for studying networks’ scalability and performance traits. Since natural systems can offer important concepts for modeling network systems, key adaptive and emergent attributes of honeybees and their societal properties are incorporated into a set of simulation models that are developed using the discrete event system specification approach. Large-scale network models are simulated and evaluated to show the benefits of nature-inspired network models.     

JiST: an efficient approach to simulation using virtual machines

Discrete event simulators are important scientific tools and their efficient design and execution is the subject of much research. In this paper, we propose a new approach for constructing simulators that leverages virtual machines and combines advantages from the traditional systems‐based and language‐based simulator designs. We introduce JiST, a Java‐based simulation system that executes discrete event simulations both efficiently and transparently by embedding simulation semantics directly into the Java execution model. The system provides standard benefits that the modern Java runtime affords. In addition, JiST is efficient, out‐performing existing highly optimized simulation runtimes. As a case study, we illustrate the practicality of the JiST framework by applying it to the construction of SWANS, a scalable wireless ad hoc network simulator. We simulate million node wireless networks, which represents two orders of magnitude increase in scale over what existing simulators can achieve on equivalent hardware and at the same level of detail. Copyright © 2005 John Wiley & Sons, Ltd.     

Testbed results of an opportunistic routing for multi-robot wireless networks

Opportunistic routing is a candidate for multihop wireless routing where the network topology and radio channels vary rapidly. However, there are not many opportunistic routing algorithms that can be implemented in a real multihop wireless network while exploiting the node mobility. It motivates us to implement an opportunistic routing, random basketball routing (BR), in a real multi-robot network to see if it can enhance the capacity of the multihop network as mobility increases. For implementation purposes, we added some features, such as destination RSSI measuring, a loop-free procedure and distributed relay probability updating, to the original BR. We carried out the experiments on a real multi-robot network and compared BR with AODV combined with CSMA/CA (routing + MAC protocol). We considered both static and dynamic scenarios. Our experiments are encouraging in that BR outperforms AODV + CSMA/CA, particularly in dynamic cases; the throughput of BR is 6.6 times higher than that of AODV + CSMA/CA. BR with dynamic networks shows 1.4 times higher throughput performance than BR with static networks. We investigate the performance of BR in the large-scale network using NS-2 simulation. We verify the effect of node density, speed, destination beacon signal and loop-free procedure. According to the large-scale simulation, the end-to-end throughput grows with the node speed.     

Simulating Robots Without Conventional Physics: A Neural Network Approach

The construction of physics-based simulators for use in Evolutionary Robotics (ER) can be complex and time-consuming. Alternative simulation schemes construct robotic simulators from empirically-collected data. Such empirical simulators, however, also have associated challenges. This paper therefore investigates the potential use of Artificial Neural Networks, henceforth simply referred to as Neural Networks (NNs), as alternative robotic simulators. In contrast to physics models, NN-based simulators can be constructed without requiring an explicit mathematical model of the system being modeled, which can simplify simulator development. The generalization abilities of NNs, along with NNs’ noise tolerance, suggest that NNs could be well-suited to application in robotics simulation. Investigating whether NNs can be effectively used as robotic simulators in ER is thus the endeavour of this work. Two robot morphologies were selected on which the NN simulators created in this work were based, namely a differentially steered robot and an inverted pendulum robot. Accuracy tests indicated that NN simulators created for these robots generally trained well and could generalize well on data not presented during simulator construction. In order to validate the feasibility of the created NN simulators in the ER process, these simulators were subsequently used to evolve controllers in simulation, similar to controllers developed in related studies. Encouraging results were obtained, with the newly-evolved controllers allowing experimental robots to exhibit obstacle avoidance, light-approaching behaviour and inverted pendulum stabilization. It was thus clearly established that NN-based robotic simulators can be successfully employed as alternative simulation schemes in the ER process.     

Pseudo-fuzzy discrete-event simulation for on-line production control

In this paper we propose an alternative use of commercial object-oriented discrete-event simulators. We attempt to bridge the intrinsic inaccuracy of simulators in modelling real systems affected by fuzziness. The strategy adopted, called pseudo-fuzzy discrete event simulation, models fuzziness through a set of several classic simulation runs to trace an output fuzzy performance function. The idea behind the approach proposed is to use the simulator as a fuzzy operator, which embeds some stochastic functions.A benchmark industrial setting has been used to build a reference simulation model and perform evaluations of the simulation strategy proposed for a specific working case.     

Large-scale simulations and performance evaluation of connected cars - A V2V communication perspective

Performance evaluation is integral to the vast majority of research on Vehicle-to-Vehicle (V2V) technology enabled connected cars. To validate ideas and concepts, researchers have been continuously striving towards the higher accuracy of simulation-based performance evaluation. However, many state-of-the-art network simulators lack comprehensive physical (PHY) layer models. More often, simplified representations of vehicular channel characteristics are used to achieve a trade-off between accuracy and performance. Vehicular channel modeling is a highly complex task because of its unique properties, for example, higher carrier frequency, rapid fluctuations in vehicular channels due to moving scatterers, and propagation in horizontal plane instead of a vertical plane with diffraction and reflection. Efficiently incorporating vehicular channel details into a single network simulator is infeasible; instead, a chain of simulation tools are used together. In this paper, we proposed a two-stage simulation framework which combines several layers of simulation tools into two distinct stages. During the first stage, a Geometry-based vehicular propagation model is used to characterize received signal strength among transmitter-receiver pairs. For this purpose, metropolitan area-wide 2.5D building geometry data and vehicular mobility traces are employed to represent the real-world environment. Subsequently, the output from the first stage is collected and fed as an input to the network simulator. Through extensive simulation-based studies, we analyze the difference between the proposed framework and standard propagation models implemented in the network simulator and their impact on the network-level performance metrics such as packet loss rate (PLR), throughput, latency, and jitter.     

基于NS-3的水声传感器网络CW-MAC协议吞吐量分析

随着微型传感器技术、无线通信技术、微电子系统制造技术以及嵌入式技术的发展,集感知、存储计算和通信能力于一体的无线集成传感器得到越来越广泛的应用,水下传感器网络就是其中的一个重要应用领域。针对水下传感器网络的特性,详细阐述基于竞争的CW-MAC协议基本思想和新型网络仿真平台NS-3,并在NS-3上搭建仿真场景,对CW-MAC协议进行模拟仿真。改变仿真节点参数,着重比较每次仿真结果得到的信道平均吞吐量。仿真结果为搭建最佳水下通信节点条件提供重要依据。     

Rapid simulator development for multicast protocol analysis

When evaluating new protocols and algorithms it is often desirable to be able to rapidly build a specialized simulator from scratch. The advantage of this approach is that software designers can avoid the often‐steep learning curve associated with using existing simulators, and that specialized simulators can have significant computational advantages over general‐purpose programs. The drawback is that simulators can be complex to develop and debug. This paper describes our experiences developing SESAME, a Java‐based simulator designed to analyze multicast protocol performance. By using an appropriate set of design patterns we were able to rapidly develop a reusable and easy‐to‐implement framework for multicast algorithm and protocol analysis. We developed several useful approaches for rapid simulation development, including a multicast visualization tool and techniques for simulating concurrent processing within a computer network that substantially simplified code development and maintenance. Owing to performance concerns, we also ran an extensive series of performance tests of different priority queue implementations. Our design approach and the lessons that we learned in developing SESAME can be applied to rapid simulation development in general. Further, our performance results suggest that by using commercial off‐the‐shelf Java development environments it is possible to obtain sufficient performance for many time‐sensitive applications such as discrete event simulation. Copyright © 2001 John Wiley & Sons, Ltd.     

Mixing It Up

Micro's editor in chief introduces the topics covered by the four articles in this general-interest issue: an interconnection network using highly integrated photonic technology; the ManySim simulation framework for future large-scale chip-multiprocessors; the SimWattch simulation framework, which integrates the Simics functional simulator with the SimpleScalar/Wattch microarchitecture simulators; and self-configuring embedded systems.     

Local interactions over global broadcasts for improved task allocation in self-organized multi-robot systems

We present a study of self-organized multi-robot task-allocation, examining performance under local and centralized communication strategies. The results extend our current understanding of the effects of communication by providing evidence that local strategies can improve system performance over centralized strategies, in terms of total task throughput as well as reduced communication overheads. The framework employed is the attractive field model, a generic model of self-organized division of labour derived from observations of ant, human and robot social systems. The framework provides sufficient abstraction to accommodate both communication strategies. Each of the studies used 16 e-puck robots in a simplified manufacturing environment where sensing and communication was realized using camera-based overhead tracking and centralized communication. In terms of task throughput, communication overhead and energy efficiency, the experimental results show that systems with restricted access to information perform better than systems with free flow of information. This suggests a potential paradigm shift where, for self-organizing systems, diminishing access to information renders a system more efficient.     

Architectures and coder design for networked control systems

In networked control systems (NCSs) achievable performance is limited by the communication links employed to transmit signals in the loop. In the present work, we characterise LTI coding systems which optimise performance for various NCS architectures. We study NCSs where the communication link is situated between plant output and controller, and NCSs where the communication link is located between controller and actuator. Furthermore, we present a novel NCS architecture, which is based upon the Youla parameterisation. We show that, which of these architectures gives best performance depends, inter alia, upon characteristics of a related non-networked design, plant disturbances and reference signal. A key aspect of our work, resides in the utilisation of fixed signal-to-noise ratio channel models which give rise to parsimonious designs, where channel utilisation is kept low. The results are verified with simulations utilising bit-rate limited channels.